So called Chireix amplifiers are examples of RF amplifiers which are based on multiple transistors with passive output network interaction which gives a high average efficiency for amplitude-modulated signals.
Such amplifiers, i.e. amplifiers based on passive output network interaction structures have a general advantage in that they need only fundamental (i.e. RF) frequency network and signal modifications; compared to single-transistor amplifiers they differ in the number of independently driven transistors. Many other types of high-efficiency amplifiers require harmonics and/or baseband modifications, features which are optional for amplifiers based on passive output network interaction structures.
However, one problem in amplifiers based on passive output network interaction structures is transistor shunt loss. This loss is due to resistive parasitics that couple the output node of the devices to ground. This problem is emphasized by the operation of multi-transistor amplifiers, which decrease the transistors' RF output currents (which is the reason for their high efficiency) at the expense of increased RF output voltages.